While many organisms are auxotrophic for Fe(III), because of the insolubility of the hydroxide (Ksp=1×10−38) [Acc. Chem. Res., Vol. 12, Raymond et al., “Coordination Chemistry and Microbial Iron Transport,” pages 183-190 (1979)] formed under physiological conditions, nature has developed rather sophisticated iron storage and transport systems. Micro-organisms utilize low molecular weight ligands, siderophores, while eukaryotes tend to utilize proteins to transport iron, e.g. transferrin, and store iron, e.g., ferritin [Trends in Biochem. Sci., Vol. 11, Bergeron, “Iron: A Controlling Nutrient in Proliferative Processes,” pages 133-136 (1986)].
Iron metabolism in primates is characterized by a highly efficient recycling process with no specific mechanism for eliminating this transition metal [Clin. Biochem., Vol. 4, Finch et al, “Iron Metabolism,” pages 5-10 (1986); Ann. Rev. Nutri., Vol. 1, Hallberg, “Bioavailability of Dietary Iron in Man,” pages 123-147 (1981); N. Engl. J. Med., Vol. 306, Finch et al, “Perspectives in Iron Metabolism,” pages 1520-1528 (1982); and Medicine (Baltimore), Vol. 49, Finch et al, “Ferrokinetics in Man,” pages 17-53 (1970)]. Because it cannot be effectively cleared, the introduction of “excess iron” into this closed metabolic loop leads to chronic overload and ultimately to peroxidative tissue damage [The Molecular Basis of Blood Diseases, Seligman et al, “Molecular Mechanisms of Iron Metabolism,” page 219 (1987); Biochem. J., Vol. 229, O'Connell et al, “The Role of Iron in Ferritin- and Haemosiderin-Mediated Lipid Peroxidation in Liposomes,” pages 135-139 (1985); and J. Biol. Chem., Vol. 260, Thomas et al, “Ferritin and Superoxide-Dependent Lipid Peroxidation,” pages 3275-3280 (1985)]. There are a number of scenarios which can account for “iron overload,” e.g., high-iron diet, acute iron ingestion or malabsorption of the metal: In each of these situations, the patient can be treated by phlebotomy [Med. Clin. N. Am., Vol. 50, Weintraub et al, “The Treatment of Hemochromatosis by Phlebotomy,” pages 1579-1590 (1966)]. However, there are iron-overload syndromes secondary to chronic transfusion therapy, e.g., aplastic anemia and thalassemia, in which phlebotomy is not an option [Iron in Biochemistry and Medicine, Vol. II, Hoffbrand, “Transfusion Siderosis and Chelation Therapy,” page 499 (London, 1980)]. The patient cannot be bled, as the origin of the excess iron is the transfused red blood cells; thus, the only alternative is chelation therapy. However, to be therapeutically effective, a chelator must be able to remove a minimum of between 0.25 and 0.40 mg of Fe/kg per day [Semin. Hematol., Vol. 27, Brittenham, “Pyridoxal Isonicotinoyl Hydrazone: An Effective Iron-Chelator After Oral Administration,” pages 112-116 (1990)].
Although considerable effort has been invested in the development of new therapeutics for managing thalassemia, the subcutaneous (sc) infusion of desferrioxamine B, a hexacoordinate hydroxamate iron chelator produced by Streptomyces pilosus [Helv. Chim. Acta, Vol. 43, Bickel et al, “Metabolic Properties of Actinomycetes. Ferrioxamine B,” pages 2129-2138 (1960)], is still the protocol of choice. Although the drug's efficacy and long-term tolerability are well-documented, it suffers from a number of shortcomings associated with low efficiency and marginal oral activity.
Although a substantial number of synthetic iron chelators have been studied in recent years as potential orally active therapeutics, e.g., pyridoxyl isonicotinoyl hydrazone (PIH) [FEBS Lett., Vol. 97, Ponka et al, “Mobilization of Iron from Reticulocytes: Identification of Pyridoxal Isonicotinoyl Hydrazone as a New Iron Chelating Agent,” pages 317-321 (1979)], hydroxypyridones [J. Med. Chem., Vol. 36, Uhlir et. al, “Specific Sequestering Agents for the Actinides. 21. Synthesis and Initial Biological Testing of Octadentate Mixed Catecholate-hydroxypyridinonate Ligands,” pages 504-509 (1993); and Lancet, Vol. 1, Kontoghiorghes et al, “1,2 -Dimethyl-3-hydroxypyrid-4-one, an Orally Active Chelator for the Treatment of Iron Overload,” pages 1294-1295 (1987)] and bis(o-hydroxybenzyl)-ethylenediaminediacetic acid (HBED) analogues [Ann. N.Y. Acad. Sci., Vol. 612, Grady et al, “HBED: A Potential Oral Iron Chelator,” pages 361-368 (1990)), none has yet proven to be completely satisfactory. Interestingly, the siderophores have remained relatively untouched in this search. Their evaluation as iron-clearing agents has not at all paralleled the rate of their isolation and structural elucidation. In fact, until recently, beyond DFO, only two of some 100 siderophores identified have been studied in animal models: enterobactin [Gen. Pharmac., Vol. 9, Guterman et al, “Feasibility of Enterochelin as an Iron-Chelating Drug: Studies with Human Serum and a Mouse Model System,” pages 123-127 (1978)] and rhodotorulic acid [J. Pharmacol. Exp. Ther., Vol. 209, Grady et al, “Rhodotorulic Acid-Investigation of its Potential as an Iron-Chelating Drug,” pages 342-348 (1979)]. While the former was only marginally effective at clearing iron, the latter compound was reasonably active. Unfortunately, both of these cyclic siderophores exhibited unacceptable toxicity, and neither possessed any oral activity. They were abandoned as there were any number of synthetic chelators with equally unsatisfactory properties from which to choose.
U.S. patent application Ser. No. 08/624,289 filed Mar. 29, 1996, the entire contents and disclosure of which are incorporated herein by reference, discloses certain 2-pyridyl-Δ2-thiazoline-4-carboxylic acids and derivatives thereof useful for the treatment of human and non-human animals in need of therapy entailing the prevention of deposition of trivalent metals and compounds thereof in their tissues, as well as the elimination of such metals and compounds from biological systems overloaded therewith.
It is an object of the present invention to provide additional novel thiazoline acids and derivatives thereof which, because of different volumes of distribution in patients and different lipophilicities than the derivatives of the prior art, provide the ability to control the pharmacokinetic properties and toxicities of the drugs.
Another object of the present invention is to provide novel pharmaceutical compositions for and methods of treatment of human and non-human animals in need of therapy entailing the prevention of deposition of trivalent metals and compounds thereof in tissues thereof, as well as the elimination of such metals and compounds from systems overloaded therewith.